1. Field of the Invention
This invention relates to a modem system, and more particularly to a modem system in which, on the signal transmitting side, digital data are subjected to PSK (phase shift keying) modulation, while, on the signal receiving side, a delay modulation is carried out to obtain the original digital data.
2. Brief Discussion of the Art
In one example of a conventional digital signal transmitting system, a digital signal is PSK-modulated on the signal transmitting side, while the signal is demodulated to obtain the original digital signal on the signal receiving side.
In such a PSK modulation system, digital signals "0" and "1" are transmitted on the variation in phase of the carrier wave, and the C/N deterioration is minimized. That is, the PSK modulation system has excellent signal characteristics and has accordingly been extensively employed for transmission of digital signals.
The PSK modulation system is often configured as a so-called "MSK system" or a so-called "DSK system".
In the MSK system, as shown in FIG. 10, when the signal is in the "mark" state, the phase is increased linearly by 180.degree. for a time slot period of the signal; and when the signal is in "space" state, the phase is decreased linearly by 180.degree. for a time slot period of the signal.
In the DSK system, as shown in FIG. 11, when the signal is in "mark" state, a time slot period of the signal is divided into two parts so that the phase is increased in two steps; and when the signal is in "space" state, a time slot period of the signal is divided into two parts so that the phase is decreased in two steps.
In the MSK system, the phase changes continuously. Therefore, the MSK system is advantageous in that the occupied frequency band-width is small. On the other hand, the DSK system is advantageous in that it is substantially not affected by multi-path fading and is therefore suitable for wide-band data transmission.
In order to demodulate a signal which is PSK-modulated, a delay detection system or a synchronous detection system are generally employed.
In the delay detection system, a signal received is divided into two parts. One of the two parts, after being delayed by one (1) or half (1/2) signal period of the modulation by a delay circuit, is supplied to a phase comparator, while the other is applied, as it is, to the phase comparator, so that the PSK-modulated signal is demodulated; i.e., the original digital signal is obtained. This will now be described in more detail. It is assumed that, in a delay detection device shown in FIG. 8(A), its input voltage Vin is cos (.OMEGA.t+.theta. (t)) where .OMEGA. is the angular frequency of the carrier, t is the time and .theta. (t) is the phase modulation function. The input voltage Vin is divided into two parts. One of the two parts is supplied to a terminal of a phase comparator 22, while the other is applied to another terminal of the phase comparator 22 after being delayed by a predetermined period of time TR by a delay circuit 21. The one of the two parts, namely, a signal Vc is: EQU Vc=Vin=cos (.OMEGA.t+.theta. (t)), and
the other, namely a signal Vd is: EQU Vd=cos (.OMEGA.(t-TR)+.theta. (t-TR))
If, in this connection, the phase comparator 22 is arranged as shown in FIG. 8(B) so that the output is proportional to the phase difference as indicated in FIG. 8(C), then the phase difference .DELTA..theta. is: EQU .DELTA..theta.=.OMEGA.TR+.theta. (t)-.theta. (t-TR)
In the above expression, it is essential that, in the MSK system or DSK system, the delay time TR is determined as follows: TR=T/2 (where T is one (1) time slot of the signal).
If .OMEGA. TR=(2n-1) .pi. or .OMEGA.=.pi./TR=(2n-1)2.pi./T, then the reference point of the phase comparison can be set at the center of the range of operation of the phase comparator.
By way of example, the case of the DSK system will be described. However, it should be noted that the following description is applicable to the case of the MSK system in the same manner.
When .theta. (t)-.theta. (t-TR)=O, .DELTA..theta.=.OMEGA.TR=(2n-1).pi..
Therefore, this point is the phase reference point in the case where there is no phase shift, and an output corresponding to the point which is shifted by .theta. (t)-.theta. (t-TR) from the reference point is obtained.
In the case of a mark-space signal, its phase function .theta. (t) is as shown in FIG. 9(A), and .theta. (t-T/2) in FIG. 9(B).
Therefore, as shown in FIG. 9(C), .theta. (t)-.theta. (t-T/2) is .pi./2 for a "mark" period, and -.pi./2 for a "space" period. Thus, an output waveform as shown in FIG. 9(E) is obtained according to an output characteristic as shown in FIG. 9(D). That is, an output obtained is 3Vo/4 for a "mark" period, and Vo/4 for a "space" period.
Accordingly, it can be determined that the signal is in a "mark" state when the output of the phase comparator 22 exceeds Vo/2, and it is in a "space" state when the output is Vo/2 or lower.
Thus far a delay detection scheme has been described. Now a synchronous detection scheme will be described. In the synchronous detection circuit, a signal received is divided into two parts, which are applied to phase comparators, respectively, and the output signal (whose frequency is equal to the carrier frequency of the signal received) of a voltage-controlled oscillator in a phase synchronization loop is supplied to one of the phase comparators while the output signal, after being phase-shifted by 90.degree., is applied to the other phase comparator, so that the original digital signal is obtained according to the output signals of the two phase comparators (cf. Trans. IECE Japan, Vol. 64-B, No. 10, 1981, GMSK Modulation System Transmission Characteristic" by Kazuaki Murota, and Kenkichi Hiraide).
As described above, in the case where the PSK-modulated signal is demodulated by the delay detection system, the signal received is divided into two parts, and only one of the two parts is delayed. Therefore, the method is advantageous in that the circuitry is simple; however, it is disadvantageous in that, in the case of a high frequency band digital signal transmission, the demodulation reliability is low. This will be described in more detail. In the delay detection system, the operating reference point is .DELTA..theta.-.OMEGA.T/2. Therefore, when the carrier angular frequency changes by .DELTA..theta., for instance, because of a temperature variation, then the operating reference point is also shifted as much as .DELTA..theta.T/2. If this change is great, then the "mark" and "space" cannot be determined according to whether or not the output level of the phase comparator exceeds Vo/2. For instance, when the carrier frequency is 1.5 GHz and the coefficient of variation with temperature of the oscillator is 10.sup.-5, then the frequency variation is 15 KHz. If, in this case, the data transmission speed is set to 32 Kbps, then T=1/32 msec, and .DELTA..theta.T/2=0.469 .pi.; that is, the shift of the operating reference point is substantially .pi./2. In practice, the operating reference point is also affected by noise and interference waves by multipath in addition to temperature variation, and is thus even further shifted. Therefore, it is often difficult to determine, by comparing the output level of the phase comparator with the predetermined reference level, whether the signal is in "mark" state or in "space" state.
A synchronous detection system is based on the reproduction of a carrier frequency by a Costas loop. Therefore, the synchronous detection system, unlike the phase detection system, is free from problems attibuted to a frequency variation, thus permitting signal demodulation with high accuracy. However, the synchronous detection system has its own limitations.
In order to obtain a signal whose frequency is equal to the carrier frequency of a signal received, it is necessary to provide a local oscillator, namely, a voltage-controlled oscillator, and a phase locked loop, thus requiring intricate circuitry and a high manufacturing cost. This is a serious problem for radio equipment on vehicles, because it is essential that the radio equipment be small in size, simple in construction and have a low manufacturing cost.